The present invention relates to methods for recovering hydrocarbons, and, more particularly, to a method for recovering hydrocarbon fractions from oil-bearing materials such as oil shale in a continuous traveling grate retort.
Large deposits of oil-bearing shale exist which have been almost entirely untouched because of the lack of an economical process for recovery of the oil. The many processes for recovery which have been proposed, either for in situ recovery or for the retorting of mined oil shale, have not satisfied the requirements of producing the oil in defined hydrocarbon fractions or at a cost comparable to conventional sources.
The oil present in oil shale is in a form called "kerogen" which is an organic waxy compound. When heated over a period of time and to an appropriate temperature, pyrolysis of the oil shale occurs to yield gaseous and liquid hydrocarbon fractions. As the temperature of the oil shale increases, the liquid products of such retorting get progressively heavier. In most retorts, a bed of oil shale is heated and the oil is evolved, condensed, and collected. Conversion of the kerogen to shale oil and its evolution or eduction from the oil shale starts at the point of introduction of heat to the bed. AS the temperature front so generated moves through the bed, the retorting process correspondingly progresses through the bed.
In a fixed bed or batch type retort, such as a vertical furnace or kiln, the initially educed products (gas and light oils) must pass through the entire bed of cooler shale before recovery is accomplished. As a consequence, condensation and revaporization occur as the temperature front moves through the various levels of the bed. Subsequently evolved products have a correspondingly shorter path through the remainder of the cooler bed. Thus the cooler shale acts as a fractionator, mist agglomerator, and mist separator or collector. This results in the concentrating of gas and light oil in the initial off-gas product, with further evolution and revaporization of the oil as the temperature of the bed increases. A gradient of light to heavy oils is produced from start to finish of the retorting process, with the heavy hydrocarbon fractions appearing at or near completion of the retorting. However, batch type retorts have not proven economically advantageous due to the limited throughputs which may be achieved in such apparatus.
Oil shale retorts, therefore, are preferably adapted for continuous operation in which a bed of crushed solid material is continuously passed through the retort to be heated and the hydrocarbon products evolved. The hydrocarbon products of the prior art continuous retorting operations, such as rotary kilns and straight line or circular traveling grate retorts, have required further treatment to obtain marketable or usable hydrocarbon fractions.